Hot stack gases which issue from a blast furnace or the like at a pressure of several atmospheres are usually passed through a cleaning or purifying device which first separates out the larger particles carried by the gas and then subjects the gas to a scrubbing operation which removes many of the water-soluble gas components and removes additional particles from the gas stream.
The above-cited earlier works describe so called Venturi or differential-pressure (annular-gap) washers which comprise a tube having a narrow waist in which is received a body that is displaceable within this tube so as to define a variable gap therewith. A sprayer is provided in the upstream end of the tube so that the turbulence and pressure drop in the tube will thoroughly scrub the remaining particles from the gas.
Such differential-pressure (annular-gap) washers are used to maintain a constant backpressure in the blast furnance. A constant backpressure is necessary for proper functioning of the furnace and can readily be maintained by the washers. At the same time such devices serve to drop the pressure so that the cleaned stack gases can be used in regenerators for heating up the charge or the air that goes into the furnace. As a rule a plurality of such annular-gap washers are used with all of their insert bodies coupled together so as to permit adjustment of the pressure drop across them to maintain the pressure in the blast furnace constant.
Such systems are relatively effective. However, they have the principal disadvantage that they waste a considerable amount of energy present in the hot pressurized stack gas. A significant amount of energy available to do work is wasted as the gases pass through the washers and expand.